1. Field of the Invention
This invention relates to an improved method for extracting aromatic hydrocarbons in high yields from mixed hydrocarbon feed streams containing the same. More particularly, this invention relates to a low-energy process for the solvent extraction of aromatic hydrocarbons from non-aromatic hydrocarbons, including naphthenic and paraffinic hydrocarbons, using as the solvent N-cyclohexyl-2-pyrrolidone, and thereafter separating the solvent from the aromatic hydrocarbons utilizing minimum high-energy distillation means. The process is particularly applicable to the separation of aromatics from suitable mixed hydrocarbon streams in the preparation of lubricating oils.
2. Prior Art
The separation of aromatic from non-aromatic hydrocarbons to recover both aromatic feedstock such as benzene, xylene, toluene and the like, and non-aromatic hydrocarbons useful as lube oils, is well-known in the art. In almost all instances these processes have been directed to the use of solvents which selectively extract the aromatics from the mixed hydrocarbons, the differences in the prior art methods being principally involved with the choice of solvent which will remove those aromatics to thereby impart the most desirable characteristics to the resulting lubricating oil, such as viscosity, color, stability and the like, by removal of as much of the aromatics as possible. Thus, one of the major objectives in the choice of a solvent is its ability to remove as many of the "undesirable" aromatics as possible to provide a lube oil with these highly desirable properties.
In addition to the selective extraction abilities of solvents, a major economic consideration in the choice of solvents and related methods is the ability of the solvent to be separated and recovered from the aromatic hydrocarbons in order that it could be recycled and reused in the extraction process. Thus, it has been a further major objective of the prior art methods to choose a solvent or class of solvents which could readily be recovered from the aromatic phase of the extraction process in the most economical way possible. These prior art solvent recovery methods, which have been characterized by the use of such solvent systems as phenols, furfural, N-methyl pyrrolidone, and the like combined with secondary techniques such as steam, or combination of solvents, have proved generally effective for the purposes intended. However, most if not all of them have been highly energy-intensive in that they in that they have required at least one, and often more, heating and distillation steps, the distillation being the most energy-costly of all. Thus, it is also a major objective in the choice of a solvent that it be recoverable in as energy-effective a manner as possible.
A summary of the prior art which represents both the conventional, energy-intensive methods, and more energy-conservative methods, can be found in European Patent Office publications Nos. 43,267 and 43,685 (1982), the prior art discussions of which are incorporated herein by reference.
One example of a "low-energy" process which is pertinent to the process of the present invention is disclosed in the above Euro. Pat. No. 43,267, in which, following a conventional extraction step with an aromatic-selective solvent to form a raffinate phase and an aromatic-rich solvent phase, the latter is cooled to further form an aromatic extract phase and a solvent phase, the solvent is recycled and the aromatic hydrocarbons are recovered. Further taught in this process is the possibility of using such solvents as N-methyl-2-pyrrolidone, and "anti-solvents" such as water, ethylene glycol, glycerine and the like in conjunction with the extraction procedure.
Euro. Pat. No. 43,685, also mentioned above, teaches a related "low-energy" process in which an aforementioned "anti-solvent" for the extracted aromatics, for example water, is added to the aromatic-rich solvent phase following extraction to promote separation of the aromatic and solvent phases.
Having regard for the above methods, it is thus an object of this invention to provide a low-energy process which will result in both highly effective selective extraction of aromatic hydrocarbons from mixed hydrocarbon streams containing the same to provide a lube oil of high quality, and at the same time a means for recovering the solvent without the expenditure of huge amounts of energy and/or equipment.